kbfsgit: use refspec.Dst("") instead of reimplementing it
kbfsgit: use refspec.Dst("") instead of reimplementing it

Suggested by jzila.

Issue: #1882

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client